1. Field of the Invention
The present invention relates to a color image forming apparatus. More particularly, the present invention relates to a color image forming apparatus that can synchronize each photosensitive body in the apparatus so as to compensate for color registration errors.
2. Description of the Related Art
An image forming apparatus (such as a color laser beam printer or a color copier) generally employs a tandem structure which emits a laser beam onto photosensitive bodies arranged by color to form separate color images. The separate color images are then overlaid upon one another to form a full-color image on a paper, either directly or through a transfer belt. FIG. 1 is an illustration of a typical tandem image forming apparatus.
As shown in FIG. 1, a color image forming apparatus 1 comprises a plurality of image forming units 10Y, 10M, 10C, 10K having a plurality of photosensitive bodies 11Y, 11M, 11C, 11K and a plurality of developing devices 12Y, 12M, 12C, 12K, a transfer unit 30 opposed to the plurality of photosensitive bodies 11Y, 11M, 11C, 11K, and a fixing device 40. In the image forming unit, if the plurality of photosensitive bodies 11Y, 11M, 11C, 11K are driven in the direction indicated by the arrows, the surface of the photosensitive bodies 11Y, 11M, 11C, 11K is charged by chargers 15Y, 15M, 15C, 15K and a laser beam L is emitted from a laser scanning unit (not shown) so as to form an electrostatic latent image of an image to be developed with a first color. For example, if a yellow Y color image is developed, the yellow developing roller 13Y develops the electrostatic latent image formed on the photosensitive body 11Y with yellow toner to form a toner image. The yellow image is transferred through a transfer roller 32Y of the transfer unit onto a paper S fed by a transfer belt 31. Likewise, magenta (M), cyan (C) and black (K) images are formed and transferred to finally form a full-color image on the paper S. The fixing device 40 fixes the toner image transferred on the paper S by heating and pressing the toner image so that it is completely fixed onto the paper S. The color image forming apparatus 1 also includes toner supply rollers 14Y, 14M, 14C, 14K, driven rollers 33a, 33b, and 33c, tension roller 34, and a transfer belt cleaning unit 35.
In the color image forming apparatus 1, it is important to accurately conform each color toner image to the plurality of photosensitive bodies 11Y, 11M, 11C, 11K so that the final image is properly registered. To this end, the conventional image forming apparatus 1 operates the plurality of photosensitive bodies 11Y, 11M, 11C, 11K using one driving source, and synchronizes the photosensitive bodies based on the initial positions of the bodies.
FIG. 2 is a perspective view showing more details of the structure of the plurality of photosensitive bodies of a conventional image forming apparatus. As shown in FIG. 2, a driving source 50 generates a driving force and transmits the force through a plurality of power transmitting members 20Y, 20M, 20C, 20K and a plurality of idle gears 23M, 23C, 23K to the plurality of photosensitive bodies 11Y, 11M, 11C, 11K. The plurality of photosensitive bodies 11Y, 11M, 11C, 11K are mounted on shafts, and the shafts are mounted on a main body supporting frame (refer to “F” of FIG. 5). The plurality of power transmitting members 20Y, 20M, 20C, 20K comprise retardation gear members 21Y, 21M, 21C, 21K and transmitting gear members 22Y, 22M, 22C, 22K. The first retardation gear member 21K of the first power transmitting member 20K is meshed with a driving gear 51 of the driving source, and the first transmitting gear member 22K of the first power transmitting member 20K is meshed with a shaft gear 16K of the first photosensitive body 11K. Accordingly, if the driving source 50 is driven, the driving force is transmitted through the first power transmitting member 20K so as to rotate the first photosensitive body 11K. The first, second, and third idle roller gears 23K, 23C, 23M are disposed between the second, the third, and the fourth power transmitting members 20C, 20M, 20Y. The driving force of the driving source 50 is transmitted through the second, third, fourth power transmitting members 20C, 20M, 20Y to the second, third, and fourth photosensitive bodies 11C, 11M, 11Y so as to rotate the second, the third, and the fourth photosensitive bodies 11C, 11M, 11Y together in the same direction.
The color image forming apparatus 1, in which a plurality of photosensitive bodies are together driven by one single driving source, is designed by assuming that the feeding speed of the paper S is regular and that the entrance signal timing of a leading end of the paper S for each color calculated from an initial signal is theoretically the same.
FIGS. 3A and 3B illustrate the operation of the photosensitive bodies of the color image forming apparatus. For convenience, FIGS. 3A and 3B depict only two photosensitive bodies 11Y, 11M. Referring to FIG. 3, if the paper S is fed to the third photosensitive body 11M driven in association with the fourth photosensitive body 11Y via the fourth power transmitting member 20Y (refer to FIG. 3B), after the paper S is fed to the fourth photosensitive body 11Y (refer to FIG. 3A), the third photosensitive body 11M should have the same phase as the fourth photosensitive body 11Y as indicated by the arrows in FIG. 3B. However, the speed is changed by minute amounts due to various issues with the image forming apparatus so that registration errors occur. One of the largest issues is that the linear velocity changes due to gear run-out. The changing linear velocity produces a different interval for each color image. FIG. 4 is a graph showing the misregistration of two color images caused by a change in linear velocity.
As shown in FIG. 4, as each photosensitive body is driven by the driving source, the linear velocity varies due to gear run-out. Additionally, the difference between each photosensitive body and the laser scanning unit, which scans with a certain speed, results in a periodic change on the paper. It is difficult to completely, mechanically remove these changes. As mechanical accuracy increases, higher costs are incurred. A color image forming apparatus should compensate for color registration errors based on the cycle of the rotation speed of the photosensitive body to increase image quality. A color image forming apparatus, in which a separate driving source is connected to separate, modularized image forming units to transmit a separate driving force to each of the plurality of photosensitive bodies, can synchronize the plurality of photosensitive bodies by a simple computer program to prevent the phase difference of each driving source. However, a color image forming apparatus, in which a plurality of photosensitive bodies are driven by one driving source, can not easily prevent registration errors.
Accordingly, there is a need for an improved apparatus for driving photosensitive bodies in a tandem image forming apparatus to prevent registration errors.